1. Field of the Invention
This invention relates to the field of electronic optical mark sense recognition apparatus for discriminating marks or holes on an illuminated surface, for entry of selected data. In particular the invention concerns a mark sense recognition apparatus especially for hand marked lottery entry forms bearing pencil or ink markings to encode selections, the apparatus being characterized by an adaptively variable threshold for distinguishing reflections of marks from the reflection of the background.
2. Prior Art
Mark sense recognition apparatus are known in connection with various data entry problems such as scanning of marked sheets or cards, bar-coded product labels and the like. Typically, marks in pencil, ink or punched holes are detected by measuring the change in reflectance of the sheet as it is moved relative to a photodetector, the timing and/or sheet position being related to the signal to provide information as to selections indicated on the sheet. The sheet or card can be illuminated in the red to near infrared band by a solid state (e.g., LED) or incandescent light source. The reflectance variations as measured by a detector (e.g., a photodiode or phototransistor) are converted into an electrical signal for each channel or track, and the signal is processed by a data detector or discriminator. Typically, at some point in the signal path, the reflectance signal is compared to a threshold for determining whether a particular reflectance variation is to be regarded as a mark or not.
In connection with scanning of sheets such as data entry forms having an array of potential mark positions which are selectively marked by a user with a pen or pencil for indicating a selection, or possibly provided with punched holes for indicating selections, particular problems are encountered. The feeding of the sheets past an optical detector, and the illumination of the potential mark positions produce a form of signal variation or noise that must be distinguished from the signals produced by marks or holes indicating selections.
The signal variations or noise are produced, for example by the vibration or "flutter" of the fed sheet, which displaces the illuminated surface of the sheet relative to the optical detector, thereby affecting signal strength during the time in which a sheet is read. The brightness of the illumination means and the sensitivity of individual optical detectors (typically photodiodes or phototransistors) can vary from unit to unit and also over time. Variations in the reflectivity of the sheet material itself also occur, and reflectivity is affected by dirt, erasures, and other factors affecting individual sheets as well as limited areas of a particular sheet.
It may be possible to overcome certain of the problems which produce variations in signal strength by using high precision feeding, illumination and detector means, and by frequently adjusting the threshold level. However, this is expensive in initial cost and in maintenance requirements. For example, it is possible to encode in parallel a pixel image of an entire sheet, thereby avoiding feed variations, and to use image processing algorithms to distinguish marked positions from unmarked ones. Such a system is disclosed in U.S. Pat. No. 4,724,307--Dutton et al. This technique avoids many of the problems associated with variations in illumination and reflectance, but is relatively expensive. It would be preferable if possible to overcome such problems in hardware, using inexpensive components.
Mark sense readers use various means to eliminate from the read signal all variations caused by effects other than marks, before the signal is processed by the data detector or discriminator. For example, the mark-receiving areas of the card or sheet are delineated with nonreflective ink (at the frequencies of interest). Temperature induced variations in component properties can be sensed and compensated, etc. The critical comparison accomplished by the routine data detector is simply then a comparison of the signal to a fixed threshold, usually defined by a fixed voltage reference. Such mark readers, however, are not well adapted to the practical problems of reading hand marked sheets, even assuming compensation. Fixed reference reading requires tightly matched electrical components, a very stable and flutter-free card feed path, a stable illumination source and detector insensitive to thermal and supply voltage variations, and typically must be carefully calibrated from time to time for optimum performance. Even with these provisos, the fixed reference mode of reading will be affected by sheet variations that cannot be eliminated in the reader, e.g., erasures, smudges, mark size variations, folds in the sheet, etc.
A number of known readers for labels or sheets attempt to resolve problems with variations in reflectivity by allowing the threshold to be varied to respond to variations between sheets or cards, or variations in illumination that cause the signal level to vary. The threshold level is made adjustable, but during the shorter term of a read cycle, the threshold level is fixed. The threshold which will be applicable to a particular read cycle can be set, for example, by a sample and hold device that sets the threshold slightly below the reflectivity level of the sheet as a whole, or perhaps at a level considered representative, such as an area of the sheet which should not contain marks. Often, this requires use of photodetectors in addition to those sensing individual channels or tracks. The objective is to cause the reader to respond to variations in reflectivity on a short term basis, as characteristic of marks, and to be insensitive to variations in the average signal level, as characteristic of component drift. Unfortunately, card flutter is a relatively short term variation.
Erased areas, smudges and folds occur frequently on hand written marked sheets, and less frequently on printed materials or bar code labels. Reading hand written sheets is also made more demanding by the fact that the size and darkness of hand written marks varies substantially among writers. Some writers darken an entire area, while others simply place an "X" or a line to indicate a mark, necessitating a very close threshold for detecting the marks. For these reasons, many of the techniques used for threshold setting in connection with reading bar codes or printed materials are less than adequate for hand written marks. The writer-induced variations aggravate problems with reader variations such as flutter, illumination variations and sheet reflectivity variation.
U.S. Pat. No. 3,747,066--Vernot et al discloses a circuit that sets a variable threshold level for recognition of printed characters based on the average light level in a region including a plurality of points being sensed as well as areas spaced laterally and longitudinally of the points. U.S. Pat. No. 4,047,023--Key et al sets the threshold by averaging the signal between areas of expected marks. U.S. Pat. No. 4,162,408--Hansen uses the level just prior to the occurrence of a potential mark. In each case the threshold is affected by variations in reflectivity outside the points of interest, i.e., the areas to which the photodetectors respond. This approach will cause the reader to respond somewhat differently to sheets which are clean vs. those which have localized darker areas, for example as is typical of erased areas, smudges and the like.
Techniques wherein a threshold level is set as a function of recently detected peaks in the read signal are disclosed, for example, in U.S. Pat. No. 3,846,623--Wefers et al; U.S. Pat. No. 4,335,301--Palmer et al; and U.S. Pat. No. 4,356,389--Quirey et al. The peaks, however, may or may not be peaks which are due to marks vs. peaks which are due to a representative sheet surface adjacent the marks, rendering such devices ineffective in instances of erasure or smudging. U.S. Pat. No. 4,196,845--Chesters attempts to improve responsiveness of detectors by wave shaping techniques. U.S. Pat. No. 4,230,265--Casaly provides a sample and hold circuit wherein the sampling is triggered at the commencement of a card read cycle. Other related disclosures can be found in U.S. Pat. No. 3,949,233--Gluck; U.S. Pat. No. 3,872,329--Dodson III; U.S. Pat. No. 3,814,944--Berger; U.S. Pat. No. 3,751,636--Coles Jr.; U.S. Pat. No. 3,692,983--Cucciati et al; and, U.S. Pat. No. 3,303,329--Fritz.
There is a need for a mark reader which is particularly adapted to reading hand written marks, which is tolerant of variations in sheet reflectivity and mechanical variations in flutter of the fed sheet, and which requires a minimum of components, preferably inexpensive and standardized components, such that a multi-channel embodiment is feasible for reading a plurality of mark tracks disposed on a sheet or card. According to the invention, these requirements are met by independent and dynamically self-tracking read circuits for each of the channels of a reader. The dynamic self-adjustment of the read circuits compensates for variations due to slip quality and mechanical flutter. No adjustments are required for any segment of the circuits, either initially or in the field, and each channel independently compensates for itself. The preferred embodiment provides both data detection and slip edge or position information, thereby eliminating external sensor needs. The circuits function well at standard optical mark sense recognition feed rates (e.g., 28 inches per second, .+-.4 ips), can be adapted readily for use at other feed rates, and can be integrated as a bipolar large scale integrated circuit.